Newly published research suggests cabbage stem flea beetle tunnelling damage, rather than larvae number, is what can indicate lodging risk in oilseed rape.

The four-year study was conducted by Dr Thomas Wilkinson, ADAS senior research consultant in crop physiology, who presented the findings at a recent roundtable discussion alongside BASF’s Matt Keane and grower Chris Eglington from Norfolk.

“From measuring various stem lodging risk parameters and CSFB damage in more than 350 plants, we’ve gained insights which can help growers to reduce the risk of lodging,” says Thomas.

“Larvae counts have often been used as the primary indicator for CSFB damage, but in this study we noted the percentage of the stem damaged by CSFB tunnelling. When OSR has reached its maximum height and is therefore most at risk of lodging, the CSFB larvae will, in most situations, have left the stem and even where there are a lot of larvae, the severity of the damage we see compared with the overall structure of the crop can range from high to low, particularly in those plants with thicker stems.

“During the study we used a lodging risk model rather than actual lodging events because it enabled us to take multiple measurements and meant we weren’t relying on the vagaries of the British weather,” he adds.

Thomas says during the research, crops with a high proportion of damage from CSFB larvae are related to reduced stem diameters – which is a key factor in determining stem strength. “Yet the CSFB tunnelling damage we saw weakened stems more than could be accounted for by the reduced stem diameter alone.

“Whether this tunnelling causes thinner stems, or thicker stems dilute CSFB tunnelling damage, we aren’t wholly sure yet, but what we do know is crops with large robust stems tend to yield well and be more tolerant to CSFB larval pressure.”

BASF Caryx

Thomas adds that the work also assessed BASF’s plant growth regulator, Caryx. “Our modelling shows Caryx reduces lodging risk from one in three, to one in five or six for a ‘typical plant’.

“Where plants had up to 25% of the stem area damaged by CSFB, Caryx increases the modelled windspeed threshold from 11.1 to 12.4 m/s, or 24.8mph – 27.7mph, showing that it reduces lodging risk in conditions typical in the UK.”

Matt, BASF agronomy manager, says the results mirror what he’s observed in the field. “It’s not unusual to find OSR crops with robust stems yielding well, even when carrying high numbers of larvae. Plants with strong branching can also compensate for some of the CSFB damage in the main stem.

“Past ADAS research demonstrates the importance of reducing leaning or lodging of the crop – lodging at flowering reduces yield by 46% (lodged flat) or by 20% (lodged to 45°) and oil reduction of up to 4% can occur.

“Therefore, while we know factors such as an over-large canopy or stormy weather can increase lodging risk, it’s important to understand how CSFB might affect this too. Upright crops maintain all-important light penetration to the lower canopy which minimises pod abortion, and so maintains yield potential.”

Plant checks

Thomas reminds that a crop may appear to be developing normally, but can still contain internal stem damage. “Opening up the stem and having a check can be a useful way of observing what’s going on inside.

“This might help you to decide how to plan future crops, for example, if you’re in a situation such as early drilling which is more likely to see higher larval pressure, is there a way to increase the robustness of your stems to dilute any potential damage?”

Canopy management is important in plants which continue to develop normally despite larval damage since they’ll have weak stems, adds Matt. “Now is the time to assess whether crops are likely to require a plant growth regulator. We recommend monitoring crops at the end of February/early March and earmarking any with a green area index (GAI) above 0.8, that’s about 50% ground cover, for an application of Caryx.

“Wait for stem extension to ensure the crop is developing normally and apply Caryx® at 0.7-1L/ha from green bud. Big crops, with a GAI of 2 or more will warrant a higher rate of 1-1.4L/ha,” he says.

On-farm experience

It was ADAS’ monitoring of Chris’ crops in Norfolk which inspired this new research since he was achieving high yields despite significant levels of larvae in his crops.

“On my farm, I sow early and believe it’s important to go with low seed numbers such as 35 plants/m2. I double roll to retain moisture and achieve even germination.

“A record high of 56 CSFB larvae per plant was recorded by ADAS in one of my OSR crops, yet that went on to produce 4.6 t/ha,” he comments.

Chris believes his low plant populations are yielding well because CSFB doesn’t appear to like the big, thick stems. “I joke that it takes a chainsaw rather than a penknife to cut them open and see what’s happening on the inside, but it’s essential to find out,” he concludes.